1. Field of the Invention
The present invention relates to a magnetic tape cassette. More particularly, it relates to a cassette casing having a three-piece structure in the magnetic tape cassette for easy molding and assembling.
2. Description of the Prior Art
The conventional magnetic tape cassettes have been assembled by connecting together with a screw a pair of half casings made of plastic material and having substantially the same shape. In the conventional tape cassette, it is not easy to prepare the half casing having no deviation from the flat plane because of the deformation caused by the molding of the plastic material. When a pair of half casings are curved in the same direction, the contacting position of a magnetic head in the A side of the tape cassette is not the same as that in the B side thereby causing a difference of sensitivity. On the other hand, when the pair of half casings are curved in a different direction, problems such as a difficulty in the running of the tape, a deviation of track, a deviation of phase etc. will occur. One reason for the occurrence of the deformation in the half casing in the molding derives from the very complicated shape of the half casings in addition to the thermal deformation and the stress deformation of the plastic material.
The inventors of this application have studied to overcome the above-described disadvantages of the conventional magnetic tape cassette and have proposed a magnetic tape cassette having a three-piece structure in which the tape cassette is separated into three pieces; one piece is made of a metallic material to increase the accuracy and the other two pieces are made of a plastic material so as to provide a nearly flat plate in shape thereby providing a simple manufacturing process and an improvement of the size. (Japanese Utility Model Application No. 126245/1977).
FIG. 1 is a schematic view of a disassembled conventional three-piece structure magnetic tape cassette. A description will be simply made with reference to FIG. 1.
The conventional tape cassette comprises an upper plate (1), an intermediate frame (2) and a lower plate (3). The upper plate (1) and the lower plate (3) are formed of a plastic material, preferably a transparent plastic material and the upper surface of the upper plate (1) and the lower surface of the lower plate (3) have substantially the same size as those of the conventional products produced under the specification. The intermediate frame is a supporting frame made by the die-casting of metal such as aluminum and having a plane upper surface and a plane lower surface parallel to each other.
Counter-sunk holes (6) are formed at the four corners and the projecting portion (7) of the upper plate (1) is formed to receive screws (8) for connecting the other two pieces together. Clearance holes (18) are formed in the intermediate frame passing through the same at the positions corresponding to the position of the counter-sunk holes (6) of the upper plate (1). Threaded holes (24) are formed in the lower plate (3) so as to be in alignment with the counter-sunk holes (6) of the upper plate and the clearance holes (18) of the intermediate frame (2), and the threaded holes at the four corners are formed in annular bosses (25) which are fitted in the clearance holes (18) of the intermediate frame (2). Annular bosses (25') are also formed around the counter-sunk holes (6) at the four corners of the upper plate (1) so that the annular bosses (25') can be fitted to the clearance holes (18) of the intermediate frame (2). (FIG. 2).
The upper plate (1), the intermediate frame (2) and the lower plate (3) are assembled together by fitting the annular bosses (25') of the upper plate (1) and the annular bosses (25) of the lower plate (3) to the clearance holes (18) of the intermediate frame (2) respectively and by screwing with tapping screws (8) into these holes to fix them in one piece as shown in FIG. 2. However, as is understood from the figure, the thickness of the walls of the outer peripheral portion (2') of the four corners in the intermediate frame (2) is very slight because the clearance holes (18) for receiving the annular bosses (25') of the upper plate (1) and the annular bosses (25) of the lower plate (3) should be formed at those parts thereby substantially reducing the strength. The formation of the thin part hinders the supply of molten plastic material during the molding operation thereby possibly providing an incompletely molded product.